The present invention relates to a method and apparatus for interpreting medical data and particularly to the identification of particular waves in an atrial pressure waveform using an ECG (electrocardiogram).
The human heart receives blood from the veins and propels it into and through the arteries. The heart has two parallel and independent systems, each consisting of an atrium and a ventricle. From their anatomical positions, the right atrium and right ventricle are known as the right heart and the left atrium and left ventricle are known as the left heart. Blood from the body returns to the right atrium through two large veins, the superior and inferior venae cavae. Return of venous blood to the right atrium takes place during the entire heart cycle of contraction and relaxation. Return of venous blood to the right ventricle occurs only during the relaxation part of the heart-pumping cycle, called diastole. Near the end of diastole, the right atrium contracts and completes the filling of the right ventricle with blood. Contractions of the right ventricle expel the blood through the pulmonary arteries into the capillaries of the lung, where the blood receives oxygen. From the lung capillaries, the blood then empties into the pulmonary veins, and in turn into the left atrium. Return of blood from the pulmonary veins to the left atrium and left ventricle proceeds in a similar manner as the return of blood from the venae cavae to the right heart cavities. Contraction of the left ventricle propels the blood into the aorta. From there, blood is distributed to all arteries of the body, including the coronary arteries, which supply the heart muscle.
Contraction of the left and right ventricles occurs simultaneously, and is called systole. The blood forced from the ventricles during systole is prevented from returning to the ventricles during diastole by valves at the openings of the aortic and pulmonary arteries. These valves consist of three semilunar (half-moon-shaped) flaps of membrane. The flaps are curved in the direction of blood flow and open readily in response to pressure in that direction. When the original pressure subsides, back pressure forces the edges of the flaps together. The tricuspid valve, situated between the right atrium and right ventricle, is composed of three triangular flaps, and the bicuspid or mitral valve, between the left atrium and left ventricle, has two such flaps. The flaps of the mitral valve remain open until the left ventricle fills with blood. When the left ventricle begins to contract, the pressure of the pumped blood closes the mitral valve.
The rhythmic beating of the heart is maintained by an orderly series of electrical discharges originating in the sinus node of the right atrium. The discharges proceed through the atrioventricular node and a bundle of neuromuscular fibers (known as the bundle of His) to the ventricles. By attaching electrodes to various parts of the body, a record of the electrical discharges can be obtained. This record is called an electrocardiogram, or ECG.
Prominent parts of an ECG are the P wave, a deflection caused by the current originating in the atrium; the QRS complex, which is caused by the electrical activity of the ventricles as they contract; and the T wave, which is caused by relaxation of the ventricles. These changes in electrical activity may, in general, be sensed using probes or electrodes attached to the exterior of the body. The fluid pressure of blood in the atrium (the xe2x80x9catrial pressure waveformxe2x80x9d) may be sensed by attaching a pressure sensor or pressure transducer to a probe (such as a catheter), and positioning that probe in the atrium. Like a standard ECG, an atrial pressure waveform has several prominent parts and these parts can be associated with or related to the prominent parts of the ECG. The prominent parts of the atrial pressure waveform include an A wave. An A wave is caused by atrial systole and follows the P wave inscribed on the ECG. The A wave has a descent X that follows this initial positive deflection and is often interrupted by a small positive deflection called the C wave, which occurs when the tricuspid valve closes. At the lowest point of the descent X, complete atrial relaxation has occurred and pressure in the right atrium begins to rise again with atrial refilling. The rise in atrial pressure during ventricular systole is called the V wave. The V wave reaches its peak prior to the opening of the tricuspid valve. The descent Y of the V wave occurs as the tricuspid valve opens and the right atrium empties into the right ventricle. Pressure in the pulmonary artery (the xe2x80x9cpulmonary artery wedge pressurexe2x80x9d) normally has a waveform similar to the waveform for the pressure in the left atrium. Although, the waveform for the pulmonary artery is delayed in transmission through the capillary vessels. A normal wedge pressure waveform has clear A and V waves, which may be detected by persons trained in reading ECGs. Descents X and Y are also clear, provided the pressure tracing is not overdamped. However, C waves are often not visible in the waveform.
Detecting the occurrence of A, V, and C waves is important in determining the maximum pressure present in the atrial and pulmonary veins and to identify the presence of prolapsed or calcified heart valves. While manual techniques exist to find these waves, they are not completely satisfactory, and automated techniques for finding C waves are not generally available. Accordingly, there is a need for improved methods of determining C waves in an atrial pressure waveform.
The present invention provides a method of determining A, C, and V waves. The method includes the acts of establishing a time reference based on R waves in an ECG. Using the R waves as a reference, a first interval in the atrial pressure waveform between a first point offset a predetermined amount ahead of a first R wave and a second point offset a predetermined amount behind the first R wave is determined. The highest peak within the first interval is then identified. This peak is the A wave.
Once the A wave is identified, a second R wave in the ECG, subsequent in time to the first R wave, is identified. Once the second R wave is determined, a second interval in the atrial pressure waveform that extends from the second point to a third point is established. The third point is positioned at a distance ahead (subsequent in time) of the second point equal to a percentage of the interval from the second point to a predetermined amount behind the second R wave. The highest peak in the second interval is the V wave.
After identifying the V wave, a third interval in the atrial pressure waveform is established. The third interval extends from the highest peak in the first interval to a fourth point. The fourth point is positioned at a distance ahead of the highest peak in the first interval (the A wave) equal to a second percentage of the distance between the highest peak in the first interval and the highest peak in the second interval (the V wave). The C wave is the highest peak in the third interval. This same process may be repeated across the different R waves in the ECG to determine the A, C, and V waves for different xe2x80x9cheart beatsxe2x80x9d or cardiac cycles.
As is apparent from the above, it is an advantage of the present invention to provide a method of determining or identifying the A, C, and V waves in a waveform. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.